As for a method for producing a 3,4-dihydro-1,2,3-oxathiazin-4-one-2,2-dioxide compound and a salt thereof, Patent Documents 1 to 3 disclose the method in which acetoacetamide-N-sulfonic acid or a salt thereof is reacted with SO3 in an inert organic solvent to cyclize and ring-close, and then the product is hydrolyzed to produce 6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one-2,2-dioxide or a salt thereof. In this method, the hydrolysis is carried out by mixing the reaction product of the ring closure process and water, and the 3,4-dihydro-1,2,3-oxathiazin-4-one-2,2-dioxide compound is obtained from an organic solvent obtained by liquid-liquid separation after the hydrolysis. Furthermore, Patent Document 1 discloses that the organic solvent obtained by the liquid-liquid separation after the hydrolysis may be purified by extraction using water or a dilute aqueous sulfuric acid.
However, when water is used for the hydrolysis of the cyclized product, the organic solvent obtained by the liquid-liquid separation after the hydrolysis is colored remarkably, so that purification loads for the target compound after the hydrolysis are increased. Furthermore, even if the organic solvent is extracted using water or the dilute aqueous sulfuric acid, hue of the organic solvent is not so improved.                Patent Document 1: Japanese Unexamined Patent Publication No. 62-56481.        Patent Document 2: Japanese Unexamined Patent Publication No. 62-129277.        Patent Document 3: Japanese Unexamined Patent Publication No. 2005-263779.        